1. Field of the Invention
This invention relates to paper feed for computer printers and more specifically to an apparatus for maintaining proper tension on a computer printer generated form as it advances through a print station by the use of cams to disengage the idler roller assembly.
2. Description of the Related Art
The majority of computer printers require printer paper feed tension for quality printing results. In printers such as Texas Instruments Model 885, printer paper feed tension for sprocket fed print media, such as perforated continuous paper forms, is performed by the coordinated action of a nip station and a paper tractor feed. The nip station surface is where the idler rollers and drive roller abut. Because the idler and drive rollers rotate in opposite directions against the print media sandwiched between, they assist the print media through the print station. The idler roller is held against the drive roller by the use of springs. The print station generally comprises either a printhead-ribbon-platten or laser-toner ink configuration and is capable of printing images or characters on the print media. The continuous paper leaving the paper tractor feed threads past the print station and between the rollers at the nip station, emerging at a tear-off bar.
During forward print media movement, the primary continuous movement is due to the paper tractor feed. Tension is maintained above the print station by the nip station which typically rotates at a speed slightly faster than the paper tractor feed.
Unfortunately, during reverse print media movement, the nip station continues to rotate faster than the paper tractor feed sprocket teeth can move the print media. As a result, print media quickly accumulates, thus creating a print media bulge between the nip station and the print station/paper tractor feed. Such form bulges often cause the form to jam.
The most common device employed to attempt to solve this problem involves using an overrunning one-way clutch on the drive shaft. The clutch drives the drive shaft only in the forward direction and disengages the drive shaft when going backward. This method has several disadvantages. Due to the viscosity of the grease in the clutches, and the mechanics of the device, not enough drag occurs to prevent paper bulges and print quality losses. Furthermore, a precision shaft is required to be used with these clutches. Both the precision shaft and the clutches are quite expensive. Yet another disadvantage involves the need for the shaft assembly to make at least a 15-20 angular minute rotation before the drive reactivates.
Another attempt to solve the problem involves putting a clutch on the idler shaft. The idler rollers are mounted directly to the shaft and the clutch disengages the idler shaft when going backward. This method locks up the idler shaft to the frame during reverse motion and thereby drags the paper across the stationary rollers. Another disadvantage to this method is that the entire shaft and rollers must be accelerated from stand still to printing speed which degrades the print quality.
Some computers embody an improvement over the above devices. Such computers employ an electronic solenoid to separate the idler and drive rollers. Nevertheless the solenoid has several disadvantages. Not only is it expensive, it takes up extra space and requires additional power for the printing mechanism. Furthermore, its operation is noisy.